Elevator systems typically include an elevator car positioned in a hoistway for carrying passengers and loads between various floors of a building. Elevator systems also include motors, which provide the requisite thrust force to move the elevator cars within the hoistways. Regenerative drives may be used to achieve desired elevator car movement and to generate electricity.
Regenerative drives may typically include a converter on the input or power utility grid side and an inverter on the motor side. In the design of a regenerative drive, power demand of the inverter is matched by an appropriate power capability of the converter. Power demands for operating elevators range from positive to negative. With positive power demands, externally generated power, such as power from a power utility grid, is used. With negative power demands, the load in the elevator drives the motor so it produces electricity as a generator. Use of the motor to produce electricity as a generator is commonly called regeneration. Regenerative drives may operate on a three-phase power input.
In addition, regenerative drives may be multilevel with a multilevel converter and a multilevel inverter. For example, a three-level regenerative drive may have a three-level converter and a three-level inverter. However, there are some issues in using a three-level converter and a three-level inverter. One issue is neutral-point voltage balancing, and another challenge is thermal management in the devices.
Accordingly, there exists a need for a control system that achieves both neutral-point voltage balancing and device thermal balancing simultaneously.